The present invention generally relates to a data companding method of recording and reproducing audio signals converted into digital signals, and to the configuration of a data compressor and a data expander which utilize the same companding method.
In recent years, the art of digitizing ;audio signals so as to effect broadcasting operations and recording and reproducing operations has advanced considerably. Digital audio broadcasting by a broadcast satellite is an example of the former operations. Also, the digital audio tape recorder (hereinafter referred to as DAT) is an example of the latter operations.
In the specification of the digital audio signals sent by satellite broadcasting, there is known as a broadcast A mode having a four-channel capacity of a standardized frequency of 32 KHz, and 14-10 near-instantaneous companding law), and a broadcast B mode having a two-channel capacity of 48 KHz and a 16-bit linear quantization in the satellite broadcasting in a case of the Japanese territory. (See the following cited literature: (1) Yoshino, Ohmi, Tsuji, Kawai: "Digital Sound Transmission System for Television Broadcasting via Satellite", The Journal of the Institute of Television Engineers of Japan, Vol 37, No. 11, PP935-941, Nov., 1983). Also, in the audio signal specification of the MAC system which is a satellite broadcasting arrangement Europe, in a standardized frequency of 32 KHz or 16 KHz, and a 14-bit linear quantization or a 14-10 bit near-instantaneous companding law variously combined are used. See the following cited literature: (2) "Specification of the system of the MAC/pocket family", PP. 85-130, EBU Technical Document 3258, Oct. 1986).
The recording modes of the audio signal specifications of the DAT (Digital Audio Tape recorder) are classified into a first mode of 48 KHz, 44.1 KHz and 32 KHz standardized frequency, 16-bit linear quantization, and two-channel capacity, and a second mode of 32 KHz standardized frequency, four-channel capacity and a 16-12 bit near-instantaneous companding law. (See the following cited literature: (3) THE DAT CONFERENCE STANDARD, "Digital Audio Tape recorder system" 1987, 6, The DAT Conference). In the recording of digital audio signals sent via a satellite broadcast by DAT, all the high-quality two-channel signals may be digitally recorded directly in the first mode of the DAT (in the case of standardized frequency of 48 KHz).
However, the signals exceeding the two channels, as in the satellite broadcast A mode, must be recorded, due to the restriction of the recording capacity, with the 14-bit data of the satellite broadcast output being data-compressed into 12 bits. Therefore, in the current DAT, the 16-12 instantaneous companding law is provided as a norm. The digital audio signals of the A mode may be recorded in the second mode (standardized frequency 32 KHz, 16-12 bit instantaneous companding law) of the DAT. However, the data of 14 bits is processed artificially as 16 bits. This is not necessarily optimal in that the 14-bit data is directly compressed into 12 bits.
As the 14-12 instantaneous companding law or the conventional example close to it, there is the 14-12 instantaneous companding law shown in the above described literature (1,) or the 13-11 instantaneous companding law shown in the following cited literature: (4) Okimi: "Transmission Techniques for Stereophonic Sound Signals", The Journal of the Institute of Television Engineers of Japan, Vol. 34, No. 4, PP322-327, Apr., 1980. In the example of the former, the vertical axis and the horizontal axis are made double in scale, with the example of the latter being provided as the basis as shown even in the same document, and the characteristic curve is similar.
As shown in the cited literature, by the use of the data companding operation, much data or information may be transmitted or recorded/reproduced with a small capacity, and the number of the channels may be increased. However, after the companding operation, a constant amount of deterioration is unavoidably caused by the signal to noise ratio (hereinafter referred to as an SN) or the like with respect to the original signal due to the data drop accompanied by the companding operation. Therefore, when the 14-12 instantaneous companding law, which is an optimal data companding method, is taken into consideration so a to record the digital audio signal (standardized frequency; 32KHz quantization bit number; 14 bits) to be transmitted in the mode, it is important to take the following two points into consideration.
Firstly, the digital audio signals to be sent by satellite broadcasting (A mode) are companded in a 14-10 near-instantaneous operation within the transmission path of the satellite broadcasting. After it has been expanded into 14 bits by the satellite broadcasting receiver, it is sent into the recording/reproducing machine and is compressed into 12 bits again. Thus, the SN deterioration amount in a L case where it has been companded by the 14-12 instantaneous companding law must not exceed the deterioration amount in the case of the 14-10 near-instantaneous companding law.
Secondly, superior companding characteristics must be retained across all the ranges of the 14-bit digital input.
The above described conventional art satisfies the conditions on a first point, but is insufficient on a second point. Namely, in order to realize the superior 14-12 instantaneous companding characteristics, it is necessary to correspondingly provide in a given relationship with the 12-bit digital data throughout the entire range of the 14-bit digital data. However, in the above described conventional art, the range (the number of the stages in the quantization) of the 14-bit digital data provided correspondingly in a given relation with the 12-bit digital data is only the range of the (0-7679) in the positive side, for example, with the data between the remaining (7680-8191) retaining the greatest value of the 12-bit digital data. The data exceeding 7679 after the companding operation is in a clip condition. The signal which has been inputted into the range has a problem in that large distortion is caused by the compression, and expansion processings of the data.
Also, the data processing between (7680 through 8191) is different in regularity from the signal processing in the other regions, in the construction of the signal processing circuit of the compression/expansion. Thus, a magnitude comparator for exclusive use, a switch for retaining the greatest value of the 12-bit digital data, and so on are additionally required, with a problem, being that the hardware scale is increased.